1. Field of the Invention
The present invention relates to high-frequency wiring structures and methods for producing the high-frequency wiring structures and, particularly, to a high-frequency wiring structure capable of reducing loss and a method for producing the high-frequency wiring structure.
2. Description of the Related Art
Increasing application of high frequencies, including microwaves and millimeter waves, for consumer products has recently been expanding the field of radio communications, such as mobile radio communications, satellite communications, and vehicle collision avoidance systems. Accordingly, higher frequencies are used for data communications, and signals with higher frequencies are processed in transmitters and receivers.
General transmitters and receivers include a printed circuit board or ceramic board, a dielectric layer formed thereon, and a fine circuit pattern (wiring pattern) formed thereon. ICs and passive components, for example, are mounted in the circuit pattern. The dielectric layer is formed by a vacuum process such as sputtering while the circuit pattern is formed by patterning using photolithography.
Japanese Unexamined Patent Application Publication Nos. 10-242599 (Patent Document 1) and 9-172107 (Patent Document 2) disclose examples of circuit patterns in transmitters and receivers that operate at high frequencies. Patent Document 1 discloses a wiring board including a transmission conductor having a bottom surface parallel to a ground layer and continuously curved side surfaces. Patent Document 2 discloses a wiring board including a transmission conductor that at least has a continuously curved surface in contact with a dielectric. Loss can be reduced according to Patent Documents 1 and 2. The dielectric used for either circuit board is photosensitive benzocyclobutene (BCB). These circuit boards are produced by forming a groove with a predetermined shape on the photosensitive BCB using photolithography and embedding a transmission conductor in the groove as signal wiring. The sectional shapes of the above transmission conductors are formed through the use of a phenomenon in which the exposure and development of photosensitive BCB allows the formation of a groove with a specific sectional shape.
For the circuit board disclosed in Patent Document 1, however, the region through which high-frequency current passes is limited to the curved parts of the transmission conductor. As a result, the cross-sectional area of the transmission conductor is small relative to the designed line width; this circuit board undesirably exhibits higher loss. On the other hand, the transmission conductor of the circuit board disclosed in Patent Document 2 has a curved surface adjacent to the dielectric and a flat top surface parallel to the ground layer, thus having sharp edges between the curved surface and the flat surface. As a result, unfortunately, high-frequency current readily concentrates on the sharp edges, leading to higher loss.